Added methods for working on env slices.

[bwplotka]

Thanks for the amazing project, hope you don't mind minor reuse and extra usability methods. 🤗

Changes

• Added methods for working on env slices.
• Added tests
• Load and Overload uses Read (code reuse)
• Avoid using return arguments raw return (antipattern)
• Exec does not pollute current process environment variables when invoked.

This allows easy use with os/exec package.

Hope this makes sense (:

Signed-off-by: Bartlomiej Plotka [email protected]